Add a testcase for strlcpy
Improve strlcpy performance on most case.

glibc strncpy is unbelievably slow if size is much larger than
strlen(src); e.g. 16K bytes vs 64 bytes.

Bug fix:
The length of src should be returned.
https://developer.apple.com/library/mac/#documentation/darwin/reference/manpages/man3/strlcpy.3.html
Add a few more comments on strlcpy